Soil salinity control relates to controlling the problem of soil salinity and reclaiming salinized agricultural land.

The aim of soil salinity control is to prevent soil degradation by salinization and reclaim already salty (saline) soils. Soil reclamation is also called soil improvement, rehabilitation, remediation, recuperation, or amelioration.

The primary man-made cause of salinization is irrigation. River water or groundwater used in irrigation contains salts, which remain behind in the soil after the water has evaporated.

The primary method of controlling soil salinity is to permit 10-20% of the irrigation water to leach the soil, be drained and discharged through an appropriate drainage system. The salt concentration of the drainage water is normally 5 to 10 times higher than that of the irrigation water, thus salt export matches salt import and it will not accumulate.

Contents 1 The soil salinity problem 1.1 Primary cause 1.2 Secondary cause 1.3 Salt affected area 1.4 Spatial variation 2 Principles of salinity control 3 Drainage systems 4 Soil leaching 5 Soil salinity models 6 See also 7 References 8 External links

[edit] The soil salinity problem

Salty (saline) soils are soils that have a high salt content. The predominant salt is normally sodium chloride (NaCl, "table salt"). Saline soils are therefore also sodic soils but there may be sodic soils that are not saline, but alkaline.

Salty soils are a common feature and an environmental problem in irrigated lands in arid and semi-arid regions. They have poor or little crop production. ^[1] The problems are often associated with high water tables, caused by a lack of natural subsurface drainage to the underground. Poor subsurface drainage may be caused by insufficient transport capacity of the aquifer or because water cannot exit the aquifer for instance, if it is situated in a topographical depression.

Worldwide, the major factor in the development of saline soils is a lack of precipitation. Most naturally saline soils are found in (semi)arid regions and climates of the globe.

[edit] Primary cause

The primary cause of man-made salinization is the salt brought in with irrigation water. All irrigation water derived from rivers or groundwater, however 'sweet', contains salts that remain behind in the soil after the water has evaporated, which may create the environmental issue.

For example, assuming irrigation water with a low salt concentration of 0.3 g/l (equal to 0.3 kg/m³ corresponding to an electric conductivity of about 0.5 dS/m) and a modest annual supply of irrigation water of 10,000 m³/ha (almost 3 mm/day) brings 3,000 kg salt/ha each year. In the absence of sufficient natural drainage (as in waterlogged soils) and without a proper leaching and drainage program to remove salts, this would lead to a high soil salinity and reduced crop yields in the long run.

Much of the water used in irrigation has a higher salt content than in this example, which is compounded by that fact that many irrigation projects use a far greater annual supply of water. Sugar cane, for example, needs about 20000 m³/ha of water per year. As a result, irrigated areas often receive more than 3,000 kg/ha of salt per year and some receive as much as 10,000 kg/ha/year.

[edit] Secondary cause

The secondary cause of salinization is waterlogging in irrigated land.

Irrigation causes changes to the natural water balance of irrigated lands. Large quantities of water in irrigation projects are not consumed by plants and must go somewhere. In irrigation projects it is impossible to achieve 100% irrigation efficiency where all the irrigation water is consumed by the plants. The maximum attainable irrigation efficiency is about 70% but usually it is less than 60%. This means that minimum 30%, but usually more than 40% of the irrigation water is not evaporated and it must go somewhere.

Most of the water lost this way is stored underground which can change the original hydrology of local aquifers considerably. Many aquifers cannot absorb and transport these quantities of water and, the water table rises leading to water logging.

Water logging causes three problems:

• the shallow water table and lack of oxygenation of the root zone reduces the yield of most crops
• it leads to an accumulation of salts brought in with the irrigation water as their removal through the aquifer is blocked
• with the upward seepage of groundwater more salts are brought into the soil and the salination is aggravated

[edit] Salt affected area

Normally, the salinization of agricultural land affects a considerable area of irrigation projects, on the order of 20 to 30%. When the agriculture in such a fraction of the land is abandoned, a new salt and water balance is attained, a new equilibrium is reached, and the situation becomes stable.

In India alone, thousands of square kilometers have been severely salinized. China and Pakistan do not lag much behind (perhaps China has even more salt affected land than India). A regional distribution of the 3,230,000 km² of saline land world wide is shown in the following table derived from the FAO/UNESCO Soil Map of the World ^[2] .

Region	Area (106ha)
Australia	84.7
Africa	69.5
Latin America	59.4
Near and Middle East	53.1
Europe	20.7
Asia and Far East	19.5
North America	16.0

[edit] Spatial variation

Although the principles of the processes of salinization are fairly easy to understand, it is more difficult to explain why certain parts of the land suffer from the problems and other parts do not, or to predict accurately which part of the land will fall victim. The main reason for this is the variation of natural conditions in time and space, the usually uneven distribution of the irrigation water, and the seasonal or yearly changes of agricultural practices.

Only in lands with undulating topography the explanation and prediction is pretty simple: the depressional areas will degrade the most.

The preparation of salt and water balances ^[3] for distinguishable sub-areas in the irrigation project, or the use of agro-hydro-salinity models ^[4] , can be helpful in explaining or predicting the extent and severity of the problems.

[edit] Principles of salinity control

Drainage is the primary method of controlling soil salinity. The system should permit a small fraction of the irrigation water (about 10 to 20 percent, the drainage or leaching fraction) to be drained and discharged out of the irrigation project. ^[5]

In irrigated areas where salinity is stable, the salt concentration of the drainage water is normally 5 to 10 times higher than that of the irrigation water. Salt export matches salt import and salt will not accumulate.

When reclaiming already salinized soils, the salt concentration of the drainage water will initially be much higher than that of the irrigation water (for example 50 times higher). Salt export will greatly exceed salt import, so that with the same drainage fraction a rapid desalinization occurs. After one or two years, the soil salinity is decreased so much, that the salinity of the drainage water has come down to a normal value and a new, favorable, equilibrium is reached.

In regions with pronounced dry and wet seasons, the drainage system may be operated in the wet season only, and closed during the dry season. This practice of checked or controlled drainage saves irrigation water.

The discharge of salty drainage water may pose environmental problems to downstream areas. The environmental hazards must be considered very carefully and, if necessary mitigating measures must be taken. If possible, the drainage must be limited to wet seasons only, when the salty effluent does inflict the least harm.

[edit] Drainage systems

Land drainage for soil salinty control is usually by horizontal drainage system (figure left), but vertical systems (figure right) are also employed.

The drainage system designed to evacuate salty water also lowers the water table. To reduce the cost of the system, the lowering must be reduced to a minimum. The highest permissible level of the water table (or the shallowest permissible depth) depends on the irrigation and agricultural practices and kind of crops.

In many cases a seasonal average water table depth of 0.6 to 0.8 m is deep enough. This means that the water table may occasionally be less than 0.6 m (say 0.2 m just after an irrigation or a rain storm). This automatically implies that, in other occasions, the water table will be deeper than 0.8 m (say 1.2 m). The fluctuation of the water table helps in the breathing function of the soil while the expulsion of carbon dioxide (CO₂) produced by the plant roots and the inhalation of fresh oxygen (O₂) is promoted.

The establishing of a not too deep water table offers the additional advantage that excessive field irrigation is discouraged, as the crop yield would be negatively affected by the resulting elevated water table, and irrigation water may be saved.

The statements made above on the optimum depth of the watertable are very general, because in some instances the required water table may be still shallower than indicated (for example in rice paddies), while in other instances it must be considerably deeper (for example in some orchards). The establishment of the optimum depth of the water table is in the realm of agricultural drainage criteria ^[6].

[edit] Soil leaching

The unsaturated zone or vadose zone of the soil below the soil surface and the watertable is subject to four main hydrological inflow and outflow factors ^[3] :

• Infiltration of rain and irrigation water (Irr) into the soil through the soil surface (Inf) : Inf = Rain + Irr
• Evaporation of soil water through plants and directly into the air through the soil surface (Evap)
• Percolation of water from the unsaturated zone soil into the groundwater through the watertable (Perc)
• Capillary rise of groundwater moving by capillary suction forces into the unsaturated zone(Cap)

In steady state (i.e. the amount of water stored in the unsaturated zone does not change in the long run) the water balance of the unsaturated zone reads: Inflow = Outflow, thus:

• Inf + Cap = Evap + Perc     or :     Irr + Rain + Cap = Evap + Perc

and the salt balance is

• Irr.Ci + Cap.Cc = Evap.Fc.Ce + Perc.Cp + Ss

where Ci is the salt concentration of the irrigation water, Cc is the salt concentration of the capillary rise, equal to the salt concentration of the upper part of the groundwater body, Fc is the fraction of the total evaporation transpired by plants, Ce is the salt concentration of the water taken up by the plant roots, Cp is the salt concentration of the percolation water, and Ss is the increase of salt storage in the unsaturated soil. This assumes that the rainfall contains no salts. Only along the coast this may not be true. Further it is assumed that no runoff or surface drainage occurs.
The amount of salts removed by plants (Evap.Fc.Ce) is usually negligibly small: Evap.Fc.Ce = 0

The salt concentration Cp can be taken as a part of the salt concentration of the soil in the unsaturated zone (Cu) giving: Cp=Le.Cu , where Le is the leaching efficiency. The leaching efficiency is often in the order of 0.7 to 0.8 ^[7] , but in poorly structured, heavy clay soils it may be less. In the Leziria Grande polder in the delta of the Tagus river in Portugal it was found that the leaching efficiency was only 0.15 ^[8] .
Assuming that one wishes to avoid the soil salinity to increase and maintain the soil salinity Cu at a desired level Cd we have:
Ss = 0 , Cu = Cd and Cp = Le.Cd. Hence the salt balance can be simplified to:

• Perc.Le.Cd = Irr.Ci + Cap.Cc

Setting the amount percolation water required to fulfill this salt balance equal to Lr (the leaching requirement) it is found that:

• Lr = (Irr.Ci + Cap.Cc) / Le.Cd .

Substituting herein Irr = Evap + Perc − Rain − Cap and re-arranging gives :

• Lr = [ (Evap−Rain).Ci + Cap(Cc−Ci) ] / (Le.Cd − Ci) ^[5]

With this the irrigation and drainage requirements for salinity control can can be computed too.
In irrigation projects in (semi)arid zones and climates it is important to check the leaching requirement, whereby the field irrigation efficiency (indicating the fraction of irrigation water percolating to the underground) is to be taken into account.
The desired soil salinity level Cd depends on the crop tolerance to salt. Data on crop tolerance can be found online: [6] .

[edit] Soil salinity models

Please help improve this article by expanding it. Further information might be found on the talk page. (October 2007)

The majority of the computer models available for water and solute transport in the soil (e.g. SWAP ^[9], DrainMod-S ^[10], UnSatChem ^[11]), are based on Richard's differential equation for the movement of water in unsaturated soil in combination with a differential salinity dispersion equation.

The models require input of soil charac­teristics like the relation between unsaturated soil moisture content, water tension, hydraulic conductivity and dispersivity. These relations vary to a great extent from place to place and are not easy to measure. The models use short time steps and need at least a daily data base of hydrological phenomena. Altogether this makes model application to a fairly large project the job of a team of specialists with ample facilities.

Simpler models, like SaltMod ^[4] , based on seasonal water and soil balances and an empirical capillary rise function, are also available. They are useful for long-term salinity predictions in relation to irrigation and drainage practices.

Spatial variations owing to variations in topography can be simulated and predicted using salinity cum groundwater models, like SahysMod.

[edit] See also

v • d • e Agricultural water management Irrigation Surface irrigation  · Tidal irrigation  · Irrigation of alluvial fans  · Irrigation statistics  · Irrigation management  · Irrigation environmental impacts Subsurface drainage Tile drainage  · Drainage equation  · Drainage system (agriculture)  · Watertable control  · Drainage research · Drainage by wells Surface water/runoff Hydrological modelling  · Hydrological transport model  · Runoff model (reservoir) Groundwater Groundwater flow  · Groundwater energy balance  · Groundwater model  · Hydraulic conductivity  · Watertable Problem soils Acid sulphate soils  · Alkali soils  · Saline soils Agro-hydro-salinity group Hydrology (agriculture)  · Soil salinity control  · Leaching model (soil)  · SaltMod integrated model · SahysMod polygonal model: Saltmod coupled to a groundwater model Related topics To be completed

[edit] References

1. ^ I.P. Abrol, J.S.P Yadav, and F. Massoud 1988. Salt affected soils and their management, Food and Agricultural Organization of the United Nations (FAO), Soils Bulletin 39.
2. ^ R.Brinkman, 1980. Saline and sodic soils. In: Land reclamation and water management, p. 62-68. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands.
3. ^ ^{a b} Drainage and Hydrology/Salinity: Water and salt balances. Lecture notes of the International Course on Land Drainage (ICLD), International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. On line : [1]
4. ^ ^{a b} SaltMod: a tool for interweaving of irrigation and drainage for salinity control. In: W.B.Snellen (ed.), Towards integration of irrigation, and drainage management. ILRI Special report, p. 41-43. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. On line : [2] or [3]
5. ^ ^{a b} J.W. van Hoorn and J.G. van Alphen (2006), Salinity control. In: H.P. Ritzema (Ed.), Drainage Principles and Applications, p. 533-600, Publication 16, International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. ISBN 90 70754 3 39.
6. ^ Agricultural Drainage Criteria, Chapter 17 in: H.P.Ritzema (2006), Drainage Principles and Applications, Publication 16, International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. ISBN 90 70754 3 39. On line : [4]
7. ^ R.J.Oosterbaan and M.A.Senna, 1990. Using SaltMod to predict drainage and salinity control in the Nile delta. In: Annual Report 1989, International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands, p. 63-74. See Case Study Egypt in the SaltMod manual : [5]
8. ^ E.A. Vanegas Chacon, 1990. Using SaltMod to predict desalinization in the Leziria Grande Polder, Portugal. Thesis. Wageningen Agricultural University, The Netherlands
9. ^ SWAP model
10. ^ DrainMod-S model
11. ^ UnSatChem model

[edit] External links

Food and Agriculture Organization of the United Nations on soil salinity US Salinity Laboratory at Riverside, California Download SaltMod from: [7] SaltMod manual online: [8] Download SahysMod from [9] SahysMod manual online: [10]

Website on soil salinity and waterlogging : [11] Articles on soil salinity and waterlogging : [12] Frequently asked questions on soil salinity and waterlogging : [13] Reports and case studies on soil salinity and waterlogging : [14] Free software on soil salinity and waterlogging : [15]